SHALLOW-WATER VS. DEEP-WATER STRATIGRAPHIC GEOMETRIES IN THE ORGANIC-RICH SHALE/MUDSTONE DEBATE

In the central Williston basin, the Bakken Formation and overlying lower Lodgepole Formation both have fine-grained, organic-rich stratigraphic units that have been interpreted sedimentologically to represent deep-water deposition in a low-energy, distal-marine environment; however, these formations display vastly different stratigraphic geometries that challenge the conventional sedimentology interpretations.

The Bakken Formation spans the Devonian-Carboniferous boundary and includes black, organic-rich (2-26% TOC) shale units. Stratigraphic characteristics strongly support deposition of all Bakken sediments in shallow-water (<30 m), as indicated by: 1) the Bakken stratigraphic position overlying a major subaerial unconformity, 2) the restriction of Bakken strata to basinal areas, 3) a layer-cake, onlap, landward-thinning stratigraphic geometry for all Bakken units, 4) gradual landward shale pinchouts that occur by intra-shale onlap and stratal thinning, not erosional truncation, 5) the absence of shale-equivalent landward deposits, 6) unequivocal evidence for shallow-water (0-5 m) middle Bakken deposition, and 7) the absence of evidence for large intra-Bakken sea-level changes.

The Bakken shallow-water stratigraphic geometry contrasts dramatically with the deep-water stratigraphic geometry in overlying lower Lodgepole strata that formed after a rapid sea-level rise of approximately 100 m. After this base-Lodgepole flooding event, carbonate deposition expanded to cover a large portion of western North America. Lower Lodgepole strata in the Williston basin are characterized by prominent sigmoidal clinoforms. In the lower Virden clinoform, argillaceous mudstone, laminated microcrystalline dolostone, peloidal-intraclastic packstone, and oolitic limestone form a 5-15 m thick shelf facies. The shelf facies transitions seaward into a thick (maximum 80 m), peloidal-mudstone shelf-margin facies, which transitions seaward into seaward-thinning (10 m to 1 m), black, organic-rich (0.5-8% TOC), argillaceous, carbonate mudstone in a basin-floor facies, informally known as the “false Bakken,” inferred to have been deposited in 140 m water depth.